AJAX :: Modal Pop Up Gray Background Not Cover Whole Page?

Use following as BackgroundCssClass of ModalPopupExtender.

.modalBackground
{
position: absolute;
z-index: 100;
top: 0px;
left: 0px;
background-color: #000;
filter: alpha(opacity=60);
-moz-opacity: 0.6;
opacity: 0.6;
}

It worked for me for the similar issue. In case that doesn't help, please let me know :)

Reference:
modal pop up gray background not cover whole page[^]

Update:

In fact, I found the question content is same as the question in the reference link. So, it should work :D

If it is still not working, just give a try by changing position:absolute; to position: fixed;